The understanding that atherosclerosis is a sub-intimal process provided the opportunity to image the disease process. Pignoli et al utilized ultrasound and measured the intima-media complex of the carotid artery (given the technical limitation of measuring the sub-intima alone) as a surrogate for the sub-intimal atherosclerotic process. Ever since the idea that measurement of the intima-media complex or IMT can provide a window into the atherosclerosis burden came to the fore, several efforts have focused on evaluating the value of IMT in research and clinical practice.
Along with identifying the association of carotid IMT (CIMT) with various atherosclerotic risk factors, the association of CIMT with prevalent and incident heart disease and stroke were then described in several studies. CIMT was and has since been used in several clinical studies as a surrogate marker to study the progression/ stabilization or regression of atherosclerosis. Although many therapies such as statins, which have shown stabilization/ regression of CIMT, have shown improved clinical outcomes as well, some therapies have shown improvement in CIMT but have had no impact on clinical outcomes. Hence, while CIMT is a good surrogate for the atherosclerotic process, it cannot replace clinical outcomes. While the association with the process of atherosclerosis is very important, data has also suggested that CIMT can help in CHD risk prediction beyond that provided by traditional CHD risk factors. In fact the 2010 American College of Cardiology Foundation/ American Heart Association guideline for assessment of cardiovascular risk in asymptomatic individuals gave CIMT a class IIa (level of evidence B) recommendation for use in asymptomatic intermediate risk individuals for cardiovascular risk assessment.
Even though no data exist that initiating preventive therapies using a strategy that incorporates CIMT/ plaque presence along with traditional risk factors will prevent events, such data does not exist with other markers such as coronary calcium score or for that matter even with the Framingham risk score. While such data will clearly be helpful, it is unlikely that any such data will emerge in the next few years. Traditionally, screening for “sub-clinical” atherosclerosis has not been covered by health benefit providers. However in 2009, the Texas legislature passed a bill that mandated the coverage of screening for sub-clinical atherosclerosis. This bill, “The Texas Heart Attack Prevention Bill” or also referred to as “Act HB1290” was signed by the state governor in June of 2009 to be implemented from September of 2009. The bill requires health benefit providers to cover the cost (up to $200) for coronary calcium scores or ultrasonography to measure the CIMT in men between the ages of 45 and 76 and women between the ages of 55 and 76. In addition, the bill requires coverage of either of these imaging studies in anyone who has diabetes or is deemed to be at intermediate risk or higher for developing CAD, as determined by the Framingham risk score. Finally, the bill allows for these tests to be repeated every 5 years.
The passage and implementation of this legislation in Texas was more recently followed by a senate bill in the State of Florida (Bill 360) which was discussed in the early part of this year. Although the “bill history” on the Florida State government Website suggests that this bill was postponed indefinitely/ withdrawn, its introduction serves to highlight that similar interest may exist in other states as well. While the merits of such legislation can and will be debated, practitioners who wish to provide CIMT screening must keep several issues in their minds.
As with any other test, but more so with CIMT, any practitioner that offers CIMT should develop a good protocol and quality control measures. The role of the sonographer in providing quality images at appropriate angles of interrogation is key given that small changes in CIMT measurements, (which can easily happen with changes in angle) can have implications on the predicted CHD risk. A good place to start (among other excellent sources available) will be publications on use of CIMT in the. These publications not only review some of the data on CIMT and its association with cardiovascular disease but in addition focus a great deal on the measurement of CIMT and its standardization. Additionally, the practitioner must not forget to screen for atherosclerotic plaque which has value above and beyond that provided by the CIMT measurement alone.
In the future, technology advances such as higher frequency ultrasound probes and 3-D ultrasound with plaque volume (and hopefully plaque characteristics) estimation may help provide additional information and refine our utilization of carotid ultrasonography in CVD risk prediction. In the meantime, while we await such advances and their implementation, using carotid ultrasound in the right patient population and with right safeguards may be of help in refining our ability to predict coronary heart disease risk.